Airborne bacteria or other microorganisms permeate the air we breath. Some of these microorganisms with which we share our environment cause disease. Medical environments, such as hospitals, have a high degree of pathogens in the air and highly susceptible, weakened patients. The existence of biological weapons of mass destruction require protection of command centers, barracks, ships, and other closed environments against biological agents. Today's modern sealed high-rise structures with central air conditioning and heating, through duct systems, need protection from the spread of disease among its occupants and from colonies of microorganisms which may live in the duct system. Today, biologic protection is necessary on the battlefield and in the workplace, the hospital and the home.
Much effort has gone into trying to destroy atmospheric pathogens with only limited success. It has long been recognized that pathogens can be destroyed in the air if they are irradiated with ultraviolet (UV) light at a wavelength of 253.7 nanometers (Germicidal Wavelength). In order for the UV light to kill microorganisms, the UV rays must directly strike the microorganisms for a sufficient time. Because of the absolute necessity for antiseptic surroundings, UV lamps of the required Germicidal Wavelength are often used in operating rooms, wards, and nurseries of hospitals.
The exposure to UV light necessary to kill microorganisms is a product of time and intensity. However, due to the dangers to humans of irradiation from wide-spread use of UV lamps, exposure to UV light has been limited by government regulations. The current occupant exposure limit (ACGIH, NIOSH standard) for 254 m ultraviolet germicidal wavelength ceiling fixtures is 6000 .mu.watts seconds/cm.sup.2 in one eight hour day. Thus, the maximum allowed intensity per second is 0.2 .mu.W/cm.sup.2. At this intensity, eight hours at the allowed exposure level is required to gain a 90% kill of Mycobacterium tuberculosis (90% kill-value =6200.mu.watts/cm.sup.2) at head height. For 100% kill using the same standard, the value is 10,000 .mu.watts/cm.sup.2, requiring 13.89 hours of exposure. The required low intensity, and resulting long exposure times, permit migration of microorganisms out of range of the UV lamp and result in accumulation of microorganisms which survive the UV lamp in the room. Increasing air circulation does not increase exposure of microorganisms. It only moves organisms past the UV lamp without sufficient exposure.
To overcome these problems there have been various attempts to circulate air passed UV sources in enclosures which acts to shield the UV irradiation from the room's occupant. Usually, such systems are free-standing, or wall or ceiling mounted devices which circulate the air in a single room through the enclosure and, accordingly, whose protection is confined to that room. See, for example, U.S. Pat. No. 5,330,722 to Pick, which discloses a germicidal air purifier which draws air through a chamber in which there is mounted an ultraviolet source which acts to kill microorganisms caught in the filter structure. Similarly, U.S. Pat. No. 5,612,001 to Arthur L. Matschke, discloses a germicidal air cleansing enclosure having an internal ellipsoid chamber which contains UV lamps along the major axis of the ellipsoid. The unit is free-standing and treats air in a single room.
While a system such as that disclosed in U.S. Pat. No. 5,612,001 to Arthur L. Matschke, may be highly effective to cleans the contents of a single room, normal air conditioning and heating ducts would continue to allow circulation of untreated air into and out of a room. This allows untreated air containing pathogens from another room, or in the duct system, to enter the room and come into contact with humans before being treated and allow a certain amount of pathogens in a room to enter the duct system prior to being treated by the free-standing unit.
Various attempts have been made to place ultraviolet light sources in duct systems to germicidally cleans fluids such as air as they pass through the duct system. See, for example, U.S. Pat. No. 5,635,133 to Glazman, U.S. Pat. No. 5,200,156 to Wedekamp and U.S. Pat. No. 5,107,687 to Candelero. Each of these patents disclose an ultraviolet irradiation source in a duct to cleanse a fluid traveling through a duct of uniform diameter. The UV source is at right angles to the duct walls and UV energy is directed at least in part along the path of fluid flow. Thus, the level of ultraviolet energy varies along the flow path. As a result, the air circulated past the UV lamps in the prior art receive an uneven distribution of ultraviolet energy and a rapid diminution of energy levels outside the immediate area of the UV source. There has, however, been no attempt to use in the duct system an irradiation chamber which uniformly distributes UV light over an extended distance along the flow path of the fluid to uniformly irradiate all fluids traveling through the duct.